In a typical wireless communication network, wireless devices, also known as wireless communication devices, mobile stations, stations (STA) and/or user equipments (UE), communicate via a Radio Access Network (RAN) to one or more core networks (CN). The RAN covers a geographical area which is divided into service areas or cells, with each service area or cell being served by a radio network node e.g., a Wi-Fi access point or a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. The service area or cell is a geographical area where radio coverage is provided by the radio network node. The radio network node operates on radio frequencies to communicate over an air interface with the wireless devices within range of the access node. The radio network node communicates over a downlink (DL) to the wireless device and the wireless device communicates over an uplink (UL) to the radio network node.
A Universal Mobile Telecommunications System (UMTS) is a third generation telecommunication network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for communication with user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for present and future networks e.g. UTRAN, and investigate enhanced data rate and radio capacity. In some RANs, e.g. as in UMTS, several radio network nodes may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural radio network nodes connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases for e.g. 4th and 5th generation networks. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a 3GPP radio access technology wherein the radio network nodes are directly connected to the EPC core network. As such, the Radio Access Network (RAN) of an EPS has an essentially “flat” architecture comprising radio network nodes connected directly to one or more core networks.
The 3GPP is currently working on standardization of the 5th generation (5G) of radio access system, also called New Radio (NR) network. An evolved architecture for the RAN is foreseen, both for the LTE Evolution and the New Radio tracks of 5G. This includes a solution where the radio network nodes such as radio base stations may be split into parts for radio network control, packet processing and radio nodes (RN) with base-band processing and radio units. An example of the new architecture is shown in FIG. 1, indicating possible interfaces and also Radio Control Nodes (RCN) and Packet Processing Node (PPN). The NR network needs to be connected to some core network that provides non-access stratum (NAS) functions and connection to communication networks outside NR, like the internet. This is here shown as a core network as specified by 3GPP.
Existing solutions rely on frequent broadcast of cell identities and other radio area identities from all radio nodes all the time. These identities can then quickly be read by wireless devices in the wireless communication network and be reported to a serving radio network node such as a RCN or RN. The serving radio network node can then identify neighbor cells and radio network nodes.
A study item for the new 5G radio access technology, entitle New Radio (NR) or New Radio access Technology (NRAT) has been started in 3GPP. It is being assumed from the beginning that NR should be designed as a single technical framework to address all usage scenarios for evolved Mobile Broadband (eMBB) and Machine Type Communication (MTC), such as massive MTC (mMTC) and ultra-reliable MTC (uMTC). The support for all these services is not expected to be standardized in the first release(s), therefore, NR is required to be future proof i.e. NR should have the property of being enhanced by the introduction of new features, the enhancement of existing features and/or the introduction of new services. LTE design can be considered future-proof, which can be acknowledged by large amount of new features that were introduced such as enhanced Inter-cell interference coordination (eICIC), Coordinated multipoint (CoMP), Demodulation-Reference Signal (DMRS), relaying, enhanced MTC (eMTC) (incl. Cat 1/0), License Assisted Access (LAA), WiFi integration, Carrier Aggregation, Dual Connectivity while still supporting multiplexing with legacy Release 8 wireless devices. In addition to these features, 3GPP has managed to introduce new services to the LTE, such as NarrowbBand Inter of Things (NB-IoT) and Vehicle to everything (V2X) communication. However, it has been observed that changes to frame structure or reference signal design were difficult or even impossible to do in a fully backwards compatible manner. This is in particular due to the fact that legacy wireless devices may expect that regarding Cell-Specific Reference Signals (CRS) which have the following characteristics:                CRSs are constantly broadcasted in every subframe;        CRSs are transmitted in every Physical Resource Block (PRB), i.e. over the whole bandwidth for each deployed carrier.        
LTE wireless devices use CRSs for various purposes including channel estimation, e.g. for data demodulation and Physical Downlink Control Channel (PDCCH) decoding, as well as for accurate Channel State Information (CSI) measurements. And while frequently transmitted reference signals (RS) across the entire carrier bandwidth are well justified for that purpose, Radio Resource Management (RRM) measurements, such as Reference Signal Received Power (RSRP)/Reference Signal Received Quality (RSRQ) could be done with less frequent signals and indeed typically wireless device implementations use just a limited number of CRSs in time and frequency to perform neighbor cell measurements.
Considering that frequently transmitted signals cause inter-cell interference and consume energy on the network side, it appears desirable to reduce the reference signals transmitted solely for the purpose of RRM measurements, i.e., when no data or control signalling need to be transmitted. That is the case e.g. when the wireless device is in Idle mode or state meaning e.g. that the wireless device is trying to find and maintain a service and connected mode means e.g. that the wireless device is or will transfer data of a service. License Assisted Access (LAA) Rel-13, and “Small Cell On/Off” (Rel-12) rely on so-called Discovery Reference Signals (DRS) which only appears once every 40, 80 or 160 ms and comprise Primary Synchronization Signal (PSS), Secondary synchronization signal (SSS) and CRS. Wireless devices supporting these features can be configured to perform their RRM measurements only based on DRS rather than on continuous CRSs. If only such wireless devices operate on a carrier, the radio network node may choose to omit the frequently transmitted CRSs. Similar optimizations should be explored for the design of reference signals used for active mode mobility measurements in NR.
Requirements for reference signals used for RRM measurements might not be the same when used in IDLE, e.g. to support cell reselection, and CONNECTED, e.g. to support beam-based mobility/handover. To support wireless device-based mobility, e.g. cell reselection, in IDLE a wireless device could rely on signals transmitted in wide beams and/or Single Frequency Network (SFN) fashion. The reason is that there is no high gain beamforming since there is no high data rates transmission in that state. These signals would also be part of the so-called “always on” signals that should be minimized in NR, especially when there are no active wireless devices. Therefore, it is interesting to reduce the amount of these signals and/or transmit them less often. The signal could be the same sync source as in LTE (e.g. PSS/SSS) and possibly not to be transmitted very often, e.g. 100 ms, so energy can be efficiently used.
On the other hand, signals to support mobility in CONNECTED mode need to rely on a higher level of beamforming and likely be transmitted more often.
One solution is that a cell ID is transmitted and used for both IDLE and CONNECTED wireless devices. Based on the cell ID the wireless device is able to read system information and know which beam reference signals (BRS) associated to that cell ID it should look for. In the solution, beams transmitted in IDLE are the same transmitted/used in Connected although, as discussed before, these would not be needed in areas/periods with no active wireless devices. As shown in FIG. 2, synchronizations signals comprise PSS/SSS and possible extended synchronization signal (ESS), similar to as defined in LTE. Based on that the wireless device is able to derive a Physical Cell ID (PCI) and as in LTE, SSS sequence occupies 6 consecutive PRBs.
The design of highly configurable Mobility Reference Signals (MRS) only transmitted “on demand” is a solution to the need for a lean design, which is future proof and energy efficient. However, it has been identified that some kind of periodic reference signals are necessary, in addition to the on demand MRSs. The latest RAN agreements from RAN2#95 state the following:
1 In connected active mode one may use non-wireless device specific RS for measurements (wireless device may not need to be aware whether the RS is wireless device-specific or non-wireless device specific).
2 The non-wireless device specific RS can be found by the wireless device without much configuration.
3 The non-wireless device specific RS encodes an identity
The problem is that the introduction of these non-wireless device specific and possibly RSs reminds some of the CRS properties needed for a change. For example, the use of a constant transmission, even though possibly transmitted sparse in time, which may lead to a system which is not lean or future proof.
At the same time, even sparser transmissions of the synchronization signals and reference signals may be needed in areas where only Idle/inactive wireless devices are present in order to enable very long Discontinuous Reception (DRX) configurations that enable network energy efficiency features. In LTE, an Idle wireless device camps on the best cell where a cell is defined by its synchronization signals, e.g. PSS/SSS. Upon detecting and synchronizing with the PSS/SSS the wireless device knows the cell ID, e.g. PCI, and is able to acquire system information so the wireless device can access the cell. Hence, PSS/SSS serves the purpose of an idle mode synchronization signal.
In LTE, the PSS/SSS is transmitted every 5 ms. However, in many scenarios, some periods of the day and/or some specific areas, cells have a quite low traffic so that the network consumes a lot of energy due to these frequent transmissions. Therefore, there is a consensus among vendors and operators that the design of NR signal(s) should allow energy efficiency mechanisms at the network side, especially in low traffic scenarios where most of the wireless devices are in sleeping state. In working group RAN2, for example, one of the agreed design principles for system information distribution in NR, for example, goes in that direction, as shown below:
. . .
2 System information broadcast should allow configurations that enable network energy efficiency, e.g. by long DTX durations.
. . .
NR signal(s) should allow energy efficiency mechanisms at the network side.
Long Discontinuous Transmission (DTX) cycles are particularly important when there are not active wireless devices to be served by a given Transmission/Reception Point (TRP). Therefore, a direct consequence of that agreement is that the periodicity of the idle mode synchronization signal denoted “xSS”, as well as the minimum system information that is broadcasted associated to the xSS, should be configurable to allow a long DTX duration e.g. 100 ms. At the same time, idle mode procedures such as cell selection, cell reselection, system information acquisition and initial access shall still be able to fulfil NR requirements.
Thus, periodicity of the idle mode synchronization signal should allow long DTX configurations e.g. 100 ms. In LTE, some steps in that direction have already been taken for unlicensed spectrum operation. Frequent transmission, e.g. every 5 ms, of idle mode signals are not allowed and sparser transmissions are utilized. Such a sparse transmission scheme has been introduced in LAA, and is also being introduced in MuLTEFire. Already from the beginning, NR should be designed to operate in unlicensed bands, as well as under other licensing schemes.
LTE allows the sparse transmission of idle signals such as in LAA and MuLTEFire. As it has been done in LTE, to fulfil the requirements on idle mode procedures with sparsely transmitted synchronization signals, additional functionality may have to be introduced on the network side. For instance, the network may provide a measurement window, similar to the Discovery Signal Measurement Timing Configuration (DMTC) window in LAA, to aid the wireless device during cell reselection, and the network then ensures that all relevant idle mode synchronization signals are transmitted in that measurement window.
A consequence of the previous described scenario is that idle mode procedures in NR, and/or inactive ‘state’, should be designed to properly operate with sparsely transmitted, e.g., every 100 ms, synchronization signals. However, this may reduce the performance in certain scenarios where the concern of energy efficiency is not the same as in the previously described case since the presence of active wireless devices will anyway enforce the network to disable the long DTX cycles. Hence, transmissions of reference signals may not be signaled in an optimum manner.